1. Field of the Invention
The present invention relates to a method for activating a hydrogenation catalyst which, following continuous and repeated use in the production of hydrogen peroxide by the anthraquinone process, has a decreased hydrogenation selectivity and an increased by-product formation. The invention also relates to a method for producing hydrogen peroxide that includes such an activation step.
2. Description of the Related Art
The chief method for industrially producing hydrogen peroxide today is known as the anthraquinone process, and involves the use of an anthraquinone as the reaction medium. The anthraquinone is generally used after dissolution in a suitable organic medium. The organic solvent used for this purpose may comprise a single solvent or may be a mixed solvent, although a mixture of two different organic solvents is typically used. The solution prepared by dissolving the anthraquinone in the organic solvent is called a “working solution.”
The anthraquinone process begins with a reduction step in which the anthraquinone within the above working solution is reduced by hydrogen in the presence of a catalyst (which reduction is referred to below as “hydrogenation”), thereby forming the corresponding anthrahydroquinone. Next, in an oxidation step, the anthrahydroquinone is oxidized with air or an oxygen-containing gas so as to convert it back to anthraquinone while at the same time producing hydrogen peroxide. An extraction step follows in which the hydrogen peroxide that was formed within the working solution is extracted, typically with water, and thereby separated from the working solution. The working solution from which the hydrogen peroxide has been extracted is then returned again to the reduction step, thus forming a cyclic process. This process, which essentially produces hydrogen peroxide from hydrogen and air, is an extremely effective process. Hydrogen peroxide is already being industrially produced using this cyclic process.
In the above-described cyclic process for producing hydrogen peroxide, because the working solution is circulated and reused, by-products such as alkyloxyanthrones and alkyltetrahydroanthraquinone epoxides that have formed due to hydrogenation of the anthraquinone and are no longer able to generate hydrogen peroxide gradually accumulate in the working solution as the production of hydrogen peroxide continues. The formation of such by-products which are incapable of producing hydrogen peroxide leads to a loss not only of the hydrogen supplied, but also of the high-cost anthraquinone. Such reactions are thus undesirable because they increase the production costs of the hydrogen peroxide.
The following methods for reclaiming and converting such by-products into anthraquinone have been described in the art. Japanese Patent Examined Publication No. S39-8806 discloses a method for treating the working solution with an alkali or an aqueous solution of an alkali. Japanese Patent Examined Publication No. S43-11658 discloses a method for treating the working solution in the reduced state with sodium hydroxide or sodium silicate at 120° C. These prior-art methods are able to revert the by-product to the original anthraquinone, but pose problems in terms of the wastewater and work efficiency associated with treating large amounts of the working solution. Also, Japanese Patent Examined Publication No. S45-19164 describes a method that involves ozone treatment followed by treatment with an aqueous solution of sodium hydroxide, then passage of the working solution through activated alumina at 70 to 75° C. Japanese Patent Examined Publication No. S49-41040 teaches a method in which the working solution is treated at 130° C. with a supported palladium catalyst in the presence of an olefin. These latter methods are capable of restoring the by-products to the original anthraquinone, yet losses are incurred due to the adsorptive removal to the catalyst of anthraquinone within the working solution. Japanese Patent Application Laid-open No. H9-278419 discloses a method in which the working solution prior to reduction is treated in the presence of a catalyst composed primarily of γ-alumina at a temperature of from 40 to 150° C. This method is able to revert by-products to the original anthraquinone without a large loss in the anthraquinone due to adsorptive removal to the catalyst. However, it does require the procurement of high-cost palladium catalyst and olefin. Because these drawbacks are factors that increase the cost of producing hydrogen peroxide, the life of the catalyst during which the catalytic activity and the selectivity for hydrogenation are sustained is important to the catalyst used for hydrogenating anthraquinone in the reduction step of the above-described cyclic process. Of these, the hydrogenation selectivity is an especially critical factor.
Catalysts that may be used to hydrogenate anthraquinones in the reduction step of the above-described cyclic process include Raney nickel catalysts, palladium black catalysts, and carrier-supported palladium catalysts. Raney nickel catalysts have a high activity, but many drawbacks. For example, they are severely degraded by trace amounts of hydrogen peroxide in the working solution, are dangerous to handle because Raney nickel is a spontaneously flammable metal, and have a low selectivity. Palladium black catalysts have an excellent activity and selectivity, but are difficult to separate from the working solution—a fatal drawback for the industrial production of hydrogen peroxide, which readily decomposes in the presence of palladium. Supported palladium catalysts, while having an activity and a selectivity which are somewhat inferior to those of palladium black catalysts, can be separated from the working solution, and are thus suitable as catalysts for the industrial production of hydrogen peroxide.
Various carrier-supported palladium catalysts have hitherto been described, including catalysts supported on such carriers as silica, alumina, silica-alumina, aluminosilicates or alkali metal carbonates. However, these do not satisfy all the conditions required of an industrial catalyst—namely, low cost, high catalytic strength, high activity and high selectivity. The carriers that are actually employed industrially are silica oxides, alumina oxides, and silica-alumina double oxides.
The inventors earlier discovered and filed a patent application for a method of preparing, as catalysts which address the foregoing needs: silica-supported palladium catalysts containing from 0.1 to 5 wt % of alkali metal (see Japanese Patent Application Laid-open No. H9-271671). These catalysts, because of their outstanding strength, activity and life, were high-performance catalysts capable of inhibiting the formation of by-products in the hydrogenation of anthraquinones.
However, even high-performance catalysts, when continuously and repeatedly used in the above-described cyclic process, undergo declines in activity and hydrogenation selectivity. A number of methods for activating such catalysts when the catalyst activity has degraded have hitherto been described in the art. For example, European Patent No. 670182-A discloses an activation method in which the degraded catalyst is treated with an oxidized working solution. However, this method is only capable of restoring catalyst activity by the desorption of hydroquinone that has deposited on the catalyst; it lacks the ability to augment the basic catalytic activity. Moreover, it does not appear to have a hydrogenation selectivity-improving effect. Japanese Patent Application Laid-open No. H9-173872 teaches a method for augmenting catalyst activity by treatment with acids such as mineral acids, sulfonic acid and oxalic acid. Although this method does have a catalytic activity-conferring effect, alkaline components that play a role in the hydrogenation selectivity end up being removed from the carrier, as a result of which a hydrogen selectivity improving effect is not observed. U.S. Pat. No. 2,925,391 discloses an activation method in which the degraded catalyst is treated at 80° C. with an aqueous solution of sodium hydroxide having a pH of 12 or above. However, strong alkali treatment does not always have an activating effect; in some cases, the properties of the catalyst carrier are compromised by such treatment, resulting in deactivation. For instance, in Comparative Example D described in Japanese Patent Application Laid-open No. H9-173872, treatment with a 5% solution of sodium hydroxide reportedly led to deactivation.
As noted above, there exists a need for the development of a catalyst activating method which confers the high activity and high selectivity that are distinctive characteristics of high-performance catalysts. In particular, there exists a strong need for a method of augmenting both the hydrogenation selectivity and activity of hydrogenation catalysts which, after continuous and repeated use in a hydrogen peroxide production plant using the anthraquinone process, have experienced a decline in hydrogenation selectivity and increased by-product formation (such a catalyst is referred to below as a “degraded hydrogenation catalyst”), without causing any damage to the catalyst itself.